1. Field of the Invention
The present invention relates to a method, device, and system for waveform shaping of signal light.
2. Description of the Related Art
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To eliminate such a limit, a regenerative repeater for digitally processing a signal is required, and it is desirable to realize such a regenerative repeater. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing in optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, timing restoration or retiming, and waveform shaping or reshaping. Of these functions, special attention is paid to the reshaping function in the present invention to provide an ultra high-speed waveform shaping device having a simple configuration by using a distributed feedback (DFB) laser in its saturated operational condition.
The most general one of conventional waveform shapers is an optoelectric (OE) type waveform shaper so designed as to once convert input signal light into an electrical signal by using a photodetector such as a photodiode, next subject this electrical signal to electrical waveform shaping by using a logic circuit, and thereafter modulate laser light by the waveform-shaped signal. Such an OE type waveform shaper is used for a regenerative repeater in a conventional optical communication system. However, the operating speed of the OE type waveform shaper is limited by an electronic circuit for signal processing, so that the bit rate of an input signal to the regenerative repeater is fixed to a low rate.
On the other hand, as an all-optical waveform shaper capable of performing all kinds of processing in optical level, there has already been proposed various ones including a nonlinear switch accompanying wavelength conversion, such as a nonlinear optical loop mirror (NOLM) or a Michelson or Mach-Zehnder interferometer, and a switch employing a saturable absorber (see Japanese Patent Application No. 10-176316 for the related art).
It is therefore an object of the present invention to provide a novel method, device, and system for waveform shaping independent of the bit rate, pulse shape, etc. of signal light. Other objects of the present invention will become apparent from the following description.
In accordance with a first aspect of the present invention, there is provided a method for waveform shaping of signal light. In this method, a distributed feedback (DFB) laser having a stop band defined as the range of wavelengths allowing laser oscillation is first provided. The DFB laser is driven so as to oscillate at a first wavelength included in the stop band. Signal light having a second wavelength not included in the stop band is input into the DFB laser.
The driving of the DFB laser is performed, for example, by supplying a constant drive current to the DFB laser.
The signal light is provided, for example, by optical pulses each having a high level and a low level. In this case, amplitude fluctuations at the high level of the signal light are suppressed in the DFB laser by the application of the present invention. This suppression effect can be optimized by adjusting the power of the signal light to be input into the DFB laser.
According to the first aspect of the present invention, the waveform shaping of signal light can be performed without the need for opto/electric conversion or electro/optic conversion, so that it is possible to provide a novel method for waveform shaping independent of the bit rate, pulse shape, etc. of signal light.
Preferably, control light having a third wavelength not included in the stop band is input into the DFB laser. The control light has a substantially constant power, for example. By inputting the control light into the DFB laser, an excess increase in noise at the low level of the signal light is suppressed. This suppression effect can be optimized by adjusting the power of the control light.
The DFB laser has an output saturation characteristic as will be hereinafter described. The signal light is subjected to waveform shaping according to the output saturation characteristic to obtain waveform-shaped light, which is output from the DFB laser.
In accordance with a second aspect of the present invention, there is provided a method for waveform shaping of signal light. In this method, signal light is divided into first signal light and second signal light. The first signal light is input into a first DFB laser having a first output saturation characteristic. The second signal light is input into a second DFB laser having a second output saturation characteristic different from the first output saturation characteristic. First waveform-shaped light output from the first DFB laser according to the first output saturation characteristic and second waveform-shaped light output from the second DFB laser according to the second output saturation characteristic are combined.
Preferably, a phase shift is imparted to the first or second waveform-shaped light so that output signal light as a difference signal between the first waveform-shaped light and the second waveform-shaped light is obtained. This phase shift is set so that the difference between a phase shift generated in the first waveform-shaped light and a phase shift generated in the second waveform-shaped light becomes xcfx80 (or an odd multiple of xcfx80). According to the second aspect of the present invention, a more rigid discrimination characteristic in relation to the output signal light can be obtained.
In accordance with a third aspect of the present invention, there is provided a device comprising a DFB laser having a stop band defined as the range of wavelengths allowing laser oscillation, and a drive circuit for supplying a drive current to the DFB laser so that the DFB laser oscillates at a first wavelength included in the stop band. Signal light having a second wavelength not included in the stop band is input into the DFB laser.
According to the third aspect of the present invention, it is possible to provide a device suitable for use in carrying out the method according to the present invention.
In accordance with a fourth aspect of the present invention, there is provided a device comprising first and second optical couplers and first and second DFB lasers. The first optical coupler divides signal light into first signal light and second signal light. The first signal light and the second signal light are input into the first and second DFB lasers, respectively. The first and second DFB lasers have first and second output saturation characteristics, respectively, wherein the first and second output saturation characteristics are different from each other. The second optical coupler combines first waveform-shaped light output from the first DFB laser according to the first output saturation characteristic and second waveform-shaped light output from the second DFB laser according to the second output saturation characteristic.
In accordance with a fifth aspect of the present invention, there is provided a device comprising an optical branch, a waveform shaper, a clock regenerator, and an optical retiming section. The optical branch divides signal light into first signal light and second signal light. The waveform shaper receives the first signal light and performs waveform shaping of the first signal light received to output resultant waveform-shaped light. The clock regenerator receives the second signal light and regenerates clock pulses according to the second signal light received. The optical retiming section receives the waveform-shaped light and the clock pulses and corrects the timing of the waveform-shaped light according to the clock pulses to output resultant regenerated signal light. The waveform shaper may be provided by the device according to the third or fourth aspect of the present invention.
The clock regenerator comprises a mode-locked laser (MLL) into which the second signal light is introduced, for example. In this case, the clock pulses may be regenerated by mode locking of the MLL according to the second signal light.
The waveform shaper comprises a nonlinear optical loop mirror, for example.
In accordance with a sixth aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting signal light, and at least one optical repeater arranged along the optical fiber transmission line. Each of the at least one optical repeater may be provided by the device according to the third, fourth, or fifth aspect of the present invention.
In accordance with a seventh aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting signal light, and an optical receiver connected to an output end of the optical fiber transmission line. The optical receiver may include the device according to the third, fourth, or fifth aspect of the present invention.
In accordance with an eighth aspect of the present invention, there is provided a device comprising a plurality of DFB lasers cascaded so that signal light is passed therethrough. Each DFB laser has a stop band defined as the range of wavelengths allowing laser oscillation. Each DFB laser is driven so as to oscillate at a first wavelength included in the stop band. The signal light has a second wavelength not included in the stop band.
According to the present invention, there is provided a method including the step of providing a DFB laser having an output saturation characteristic. Signal light is input into the DFB laser. As a result, the signal light undergoes waveform shaping according to the output saturation characteristic to obtain waveform-shaped light, which is output from the DFB laser.
According to the present invention, there is provided a method including the step of providing a DFB laser oscillating at a first wavelength. Signal light having a second wavelength different from the first wavelength is input into the DFB laser. The power of the signal light is adjusted so that the signal light undergoes waveform shaping in the DFB laser.
According to the present invention, there is provided a device comprising a DFB laser and a drive circuit for supplying a drive current to the DFB laser so that the DFB laser oscillates at a first wavelength. Signal light having a second wavelength different from the first wavelength is input into the DFB laser. The power of the signal light is adjusted so that the signal light undergoes waveform shaping in the DFB laser.
In the present invention as described above, the DFB laser oscillates in a single mode. By using this, signal light having a wavelength different from the laser oscillation wavelength of the DFB laser in its oscillating state is input into the DFB laser, thereby performing waveform shaping. However, the present invention is not limited by the use of the DFB laser, but any other lasers such as a semiconductor laser diode and a gain-clamped optical amplifier may be used. That is, optical pulses or signal light having a wavelength different from the laser oscillation wavelength of a laser in its oscillating state are/is input into the laser to thereby obtain a waveform shaping effect to the optical pulses or the signal light. For example, in a laser oscillating in multiple modes, such as a Fabry-Perot laser diode, a plurality of laser oscillation wavelengths are present, so that the signal light to be subjected to waveform shaping has a wavelength different from these laser oscillation wavelengths.
In accordance with another aspect of the present invention, there is provided an optical waveform shaping method comprising the steps of supplying a current to a laser diode so that the laser diode emits laser light, and inputting light having a wavelength different from the wavelength of the laser light emitted from the laser diode, into the laser diode to thereby perform optical waveform shaping.
In accordance with a further aspect of the present invention, there is provided an optical waveform shaping device comprising a laser diode, current supplying means for supplying a current to the laser diode so that the laser diode emits laser light, and light inputting means for inputting light having a wavelength different from the wavelength of the laser light emitted from the laser diode, into the laser diode.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.